Field of the Invention
Embodiments of the present invention relate generally to a casing exit tool. More specifically, the embodiments relate to a tool capable of milling a casing and drilling a formation in a single trip.
Description of the Related Art
In well construction and completion operations, a wellbore is formed to access hydrocarbon-bearing formations (e.g., crude oil and/or natural gas) by the use of drilling. Drilling is accomplished by utilizing a drill bit that is mounted on the end of a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, and/or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a string of casing is lowered into the wellbore. An annulus is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annulus with cement. The casing string is cemented into the wellbore by circulating cement into the annulus. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
In some production operations, it may be desirable to form a lateral wellbore, or sidetrack wellbore, relative to the cased wellbore in order to enhance the efficiency of production. Sidetrack drilling is a process which allows an operator to drill a primary wellbore, and then drill an angled lateral wellbore off of the primary wellbore at a chosen depth. Generally, the primary wellbore is first cased with the string of casing and cemented. Then, a tool known as a whipstock is positioned in the casing at the depth where deflection is desired. The whipstock is specially configured to divert a casing exit tool in a desired direction in order to mill a window in the casing and drill a lateral wellbore in the formation.
Generally, cutting structures suitable for drilling rock formations are not suitable for milling steel casing, and vice versa. For example, cutting structures suitable for milling steel casing, such as carbide, are durable and may significantly deform while drilling rock formations. As such, carbide may not effectively drill rock formations. Conversely, cutting structures suitable for drilling rock formations, such as polycrystalline diamond compact (PDC), are brittle and may chip while milling steel casing. As such, PDC may not effectively mill steel casing. Accordingly, current casing exit tools having materials for both drilling rock formations and milling steel casing are susceptible to jamming in the casing. Conventionally, this challenge is overcome by making multiple trips into the wellbore. For example, a window mill, equipped with materials suitable for cutting steel, is lowered into the primary wellbore solely to form the window in the casing. Then, the window mill is removed from the primary wellbore and replaced by a drill bit equipped with materials suitable for drilling the rock formation. The drill bit passes through the window formed by the window mill and drills the lateral wellbore. However, making multiple trips into the wellbore is expensive and time-consuming.
Thus, there is a need for a casing exit tool that can cut the casing and the formation in a single trip.